This invention relates generally to photometric apparatus for spectroscopic analysis of samples in solution. More particularly, the present invention relates to photometric apparatus having a flow cell for aqueous solutions.
A range of optical flow cells have been developed for absorption spectroscopy applications in the ultraviolet, visible and infrared regions of the light spectrum. As defined by the Beer-Lambert""s Law, absorbance (A) of light by a sample is proportional to the chromophore concentration (c), the molar absorption coefficient (xcex5), and the optical path length (1)
A=xcex5lc=log(I/I0) (Beer""s Law)
where I is the transmitted light power through the cell with sample solution and I0 is the light power transmitted trough the sample with a reference solution. In oceanography, the absorbtion of light (a) is defined as a=2.303A, where A is the absorbance of light and the optical path length (1) is specified to be 1 meter.
There are two ways by which the sensitivity of an optical sensor cell can be increased. First, the intrinsic noise of the spectrophotometer used could be reduced. For example most spectrophotometers exhibit typical noise levels equivalent to milli absorbance units (mAU). However, noise levels in spectrophotometers used in liquid chromatography are typically 100 (AU or less. The second approach is to increase the optical path length of the sample cell. However, using conventional technology, it is difficult to transmit a collimated light beam for extended distances in fluids without either very substantial light loss or, the use of advanced and very expensive collimating optics. This problem is exacerbated when the diameter of the cell must be as small as possible, in general, a requirement for most flow-through detector applications.
To overcome this problem, a light guiding flow cell is formed when an analyte solution functions as the core of a fluid filled light waveguide. Similar to optical fibers, light is confined within the (liquid) core by total internal reflection at the core/wall interface. Such flow cells are particularly suitable when combined with optical fibers for light transfer, enabling the design of a flexible sensor system. A number of flow cells with long optical path lengths have been designed for absorbance, fluorescence and Raman spectroscopy.
Such flow cells can be divided into two types on the basis of the light guiding effect and practical observations. Type I flow cells rely on the principle that the core fluid is in direct contact with the wall material (cladding) having a lower refractive index than the core fluid (U.S. Pat. Nos. 5,184,192, 5,416,879, 5,444,807, 5,604,587, and 5,608,517). Typical wall materials used for aqueous solutions include a copolymer of 2,2 bis trifluoromethyl-4,5 difluoro-1,3 dioxole with tetrafluoroethylene (Dupont xe2x80x9cTeflon AFxe2x80x9d). Teflon AF has a refractive index between 1.29 and 1.31, is chemically very inert and transparent within the 200 nm to 2000 nm spectral range. Because Teflon AF could be used to coat the internal surface of e.g. glass tubing and later could be drawn directly to tubing, it created a number of useful opportunities in the development of flow cells with long optical path lengths.
In Type II flow cells, the low refractive cladding material is not in direct contact with the core fluid, but separated by a transparent high-refractive index wall which does not interfere with the waveguide properties of the cell. The early development of waveguide sample cell technology was made difficult by the absence of a suitable cladding material which possessed a refractive index lower than that of water (n=1.33), a most commonly used solvent. This problem was originally solved by using a bar quartz capillary suspended in air. In this arrangement, light would be reflected at the outer air/glass interface. However, light transmission was found to be strongly dependent on the cleanliness of the external cell surface. Ambient dust and fingerprint contamination could easily degrade light transmission and thus the reproducibility of the analytical measurements. Tiny cracks could develop at the external surface resulting in a brittle, easily broken capillary cell. With the availability of TEFLON AF, the outside surface of a glass or fused silica capillary cell could be coated with Teflon AF producing a similar effect. The advantage of this configuration was that the total reflection would occur at the fused silica wall/Teflon AF interface and cell surface contamination could not alter the waveguide properties of Type II cells. Moreover, the fused silica tubing used in the Type II Liquid Waveguide Capillary Cell (LWCC) acted like a backbone, providing physical stability to the cell with the Teflon AF coating protecting its external surface from mechanical crack formation. The tubing could be made with a very thin wall and spooled if required (U.S. Pat. Nos. 5,416,879, 5,444,807, and 5,604,587). The hydrophilic surface of the inner silica surface reduced internal air bubble formation, which is a major problem of small diameter Type I cells, where the hydrophobic Teflon AF tends to trap air bubbles at the inner cell wall.
There are two major draw backs with the current flow cell technology and usage of single path length flow cells in general. First, all flow cells currently built are designed for low volume applications, such as liquid chromatography (LC), high pressure liquid chromatography (HPLC), or fluid injection analysis (FIA). Particles and air bubbles are easily trapped in such cells, making them difficult to use for routine sensitive laboratory or process type analysis. Further, following Beers law, the concentration of a sample is proportional to absorbance of the chromophore, which is the logarithm of the incident, I0, and transmitted light, (I) through the sample. Although the sensitivity of the measurement can be increased by increasing the optical path length, l, of the flow, cell, still the fact that the concentrationxe2x80x94intensity relationship is logarithmic, severely limits the dynamic range of a measurement. This contrasts to for example fluorescence measurements, where the relationship of sample concentration and emission intensity is linear, thus exhibiting a far higher dynamic range.
Ideally, the optical path length of a sample cell for absorbance-based flow cell should be changeable to allow for a higher dynamic range. A typical area, where a larger dynamic range than available from a single sample cell is required is found in the field of oceanography. Routinely, the concentration of colored dissolved organic matter (CDOM), which is a significant component of the bulk absorption of light in coastal waters, is determined.
The spectral absorption of CDOM is frequently an important element of bio-optical models and remote sensing algorithms for near shore waters. Traditional methods of measuring the absorption of dissolved materials require special handling and storage prior to measurement using expensive laboratory spectrophotometers. Thus, the availability of CDOM absorption measurements are often scarce or totally lacking, particularly in the optically complex and CDOM rich environment of river-dominated coastal margins. This lack of CDOM measurements limits the ability to derive appropriate regional-to-global scale mathematical color models for chlorophyll pigments and primary productivity in fresh and sea water. CDOM concentrations vary significantly between open ocean samples (0.007 mxe2x88x921 at 380 nm) and high turbidity freshwater environments with absorption as high as 10-20 mxe2x88x921 at 380 nm. This requires a higher dynamic range than a detection system based on a single path length flow cell can provide.
Briefly stated, the invention in a preferred form is a photometric detection system which comprises a light source, a photometric detector, and a flow cell assembly. The flow cell assembly includes a tubular liquid core waveguide having inlet and discharge ends for receiving and discharging a flow of the sample fluid. A first coupling device optically couples the light source to the waveguide at a first position along the length of the waveguide. A second coupling device optically couples the detector to the waveguide at a second position along the length of the waveguide. The first and second positions define an optical path length within the waveguide. The second position is variable relative to the first position such that the optical path length is selectively varied.
The second coupling device may comprise a plurality of collector optical fibers with the light receiving end of each collector optical fiber coupled to the waveguide at a collection position longitudinally spaced from the collection position of each other collector optical fiber. Each of the light emitting ends of the collector optical fibers is optically coupled to an input port of an optical switch. An outlet port of the optical switch is optically coupled to the photometric detector. The optical switch provides for selectively coupling any one of the optical path lengths to the detector. In one alternative, the first coupling device comprises a plurality of emitter optical fibers with the light emitting end of each collector optical fiber coupled to the waveguide at an emission position longitudinally spaced from the emission position of each other collector optical fiber. Each of the light receiving ends of the emitter optical fibers is optically coupled to an output port of an optical switch. An input port of the optical switch is optically coupled to the light source. In a second alternative, the first coupling device and the second coupling device both comprise a plurality of optical fibers having one end coupled to an optical switch.
In one embodiment of the flow cell assembly, first, second, third and fourth collector optical fibers are coupled to the waveguide at collection positions longitudinally spaced 2 cm, 10 cm, 50 cm and 200 cm, respectively, from the first position. Preferably, the length of the waveguide is 200 cm and the collection position for the fourth collector optical fiber is proximate to the discharge end of the waveguide. In a second embodiment of the flow cell assembly, first, second and third collector optical fibers are coupled to the waveguide with the collection position for the third collector optical fiber being proximate to the discharge end of the waveguide.
In a third embodiment of the flow cell assembly, the second coupling device comprises a single collector optical fiber. The light receiving end of the collector optical fiber is longitudinally moveable within the waveguide. A drive mechanism engaged with the collector optical fiber provides a means of moving the collector optical fiber within the waveguide. The collector optical fiber includes a hermetically sealed buffer coating. A polyimide coating may be disposed on the buffer coating. Alternatively, the light emitting end of the emitting optical fiber may be moveable within the waveguide or both the emitting optical fiber and the collector optical fiber may be moveable within the waveguide.
It is an object of the invention to provide a photometric detection system having a greater dynamic range than is available from conventional systems.
It is also an object of the invention to provide a flow cell assembly for a photometric detection system having multiple optical path lengths.
Other objects and advantages of the invention will become apparent from the drawings and specification.